As the application of lithium-ion secondary batteries to electric vehicles and hybrid electric vehicles advances, the necessity for higher energy density increases. At present, while LiCoO2 and the like are put to practical use as the positive electrode material, to increase the capacity of the lithium-ion secondary batteries, a higher-capacity positive electrode active material is indispensable.
As a positive electrode material satisfying such a demand, in recent years, a lithium excess layered manganese-nickel positive electrode represented by zLi2MnO3-(1−z)LiMO2 has widely been studied. Here, M stands for a transition metal, and zLi2MnO3-(1−z)LiMO2 can also be represented as a general formula Li(LiaMnbMc)O2. Since it is difficult for Mn to take a valence higher than quadrivalence, normally, Li2MnO3 having only quadrivalent Mn is electrochemically inactive. However, by charging it up to not less than 4.6 V with reference to lithium, a reaction occurs in which oxygen is desorbed simultaneously with lithium, which makes it electrochemically active. For this reason, Li2MnO3 exhibits a capacity of not less than 200 mAh/g by forming a solid solution with LiMO2. However, since the desorption reaction of oxygen is irreversible at the time of the initial charging, a high irreversible capacity is caused, which necessitates an excessive weight of negative electrode active material.
To reduce the initial irreversible capacity, an attempt has been made to previously remove, by using an acid such as nitric acid, a certain amount of lithium and oxygen that cause the irreversible capacity. However, with the above-described method, a surface structure of the active material is destroyed by the acid and a cycle characteristic of the battery is degraded as a consequence thereof.